Outside of Undead Nightmare, Red Dead Redemption is slow as molasses. It does not compare favorably to Activision's Gun, which inspired it. That game has a better plot, better mechanics, and a great pace.
As far as open world design I do not think anything by Rockstar has held a candle to Bully. The shear density of interactivity in its world and polish of gameplay mechanics is unrivaled.
However, I am most in awe of their artistic embellishment of 80's Miami in Grand Theft Auto: Vice City.